Technician Question of the Week T2A01 Repeater Offsets

Let’s start by defining frequency offset. Remember, a repeater receives your signal on one frequency and retransmits it on a different frequency. This is necessary for simultaneous retransmission – a repeater cannot simultaneous retransmit on the same frequency it is receiving. The difference between the repeater’s receive frequency and its transmit frequency is called the offset. So, the offset is a frequency value in units of hertz, kilohertz, or megahertz.

And when determining this difference between frequencies, the order matters! Let’s establish the following convention, in agreement with terminology used in the ARRL Repeater Directory. Let’s call the frequency that the repeater transmits the Output Frequency. Let’s call the frequency the repeater receives the Input Frequency. Please note that the Output Frequency is the one to which you listen or monitor, and the Input Frequency is the one on which you transmit to the repeater.

To properly compute the offset you must subtract the Output Frequency from the Input Frequency, or:

Input – Output = Offset

From the repeater user’s perspective, you might also think of that “Transmit – Listen = Offset.”

So, a frequency offset may be a positive or a negative value. If the repeater’s Input Frequency is higher than Output Frequency the Offset is a positive value. If the Input Frequency is lower than the Output Frequency, the Offset is a negative value.

Most repeaters will operate with a standard offset magnitude, either positive or negative in value. But be aware that sometimes the standard values are not adhered to, so don’t assume the standard in all repeater cases.

The standard offset magnitude is determined by the frequency band on which the repeater is operating. The following lists the standard offsets for bands:

70 cm Band +/- 5.0 MHz

1.25m Band +/- 1.6 MHz

2m Band +/- 600 kHz (+/- 0.6 MHz)

10m Band – 100 kHz (-0.1 MHz)

Examples:

On the 2m band suppose a repeater has an Input Frequency of 146.250 MHz and an Output Frequency of 146.850 MHz. The offset is 146.250 – 146.850 = -0.6 MHz, or -600 kHz. This is a negative standard offset.

On the 70cm band, suppose a repeater has an Output Frequency of 442.600 MHz and uses a standard positive offset. The Input Frequency is 447.600 MHz. [447.600 – 442.600 = 5.0 MHz]

The answer to Technician question T2A01, “What is the most common repeater frequency offset in the 2 meter band?” is B: plus or minus 600 kHz.